As is generally known, video transmissions, i.e., the transmission of video data, may occur over the air (E.g., broadcast television), over cable transmission links, or over satellite transmission links. Regardless of the type of video transmissions, each video transmission includes a carrier frequency on which video data and audio data are carried. A receiving television unit, or computer equipped with a television tuner, receives the video transmissions via an antenna (for over the air transmissions), a set top box (for cable transmissions), or a satellite dish and set top box (for satellite transmissions). The television unit demodulates the carrier frequency of a selected video transmission (i.e., the channel selected by the viewer) to recapture video data and audio data. The video and audio data are processed and subsequently rendered visible and audible for the viewer.
The television unit, or television tuner, includes a decoder that has an adjustable narrow band pass filter. The narrow band pass filter attenuates the signals of non-selected channels (i.e., other video transmissions) and passes the signals of the selected channel to a video decoder. For cable television, channels are spaced at approximately six MHz (6 MHz), such that the band pass filter of the tuner needs to be much less than 6 MHz (E.g., in the range of a few hundred KHz). While the channels are spaced at approximately 6 MHz, there are currently three different standards for channel identification. The first standard is the standard frequencies (STD), which transmits on the standard off-air frequencies for channels 2 through 6 and 7 through 13. For other channels, the STD standard uses supplemental channels that are 6 MHz increments down from channel 7 (175.25 MHz) to 91.25 MHz (channels 14-22 and 95-99) and upwards from channel 13 (211.25 MHz).
The second cable television channel identification standard is the harmonic related carriers (HRC). The HRC standard transmits on picture carrier frequencies that are multiples of 6.0003 MHz and starts at 54 MHz. It involves frequency displacement of 1.25 MHz on all standard and supplementary channels except channels 5 and 6, where the displacement is 0.75 MHz. The third cable channel identification standard is the incremental related carriers (IRC). The IRC standard transmits on picture carrier frequencies starting at 55.25 MHz and increments each channel by 6 MHz.
Cable companies utilize one of the three standards. The set top boxes provided to customers by the cable companies include the tuner that has been hard coded to center the band pass filter based on the particular standard. When configured in this manner, if a tuner receives a signal that was transmitted under a different standard, the tuner would attenuate the signal such that the video quality would be poor at best.
With the current deregulation of communications, it is conceivable for a consumer to receive cable broadcasts from cable carriers that use different channel identification plans. For a consumer to receive a high quality video picture from cable companies using different channel identification plans, the consumer would need to adjust its television tuner, or cable set top box, to identify which channel identification plan is being used. While many television tuners are equipped with auto fine tuning (AFT) their range is limited to 100 to 200 kilohertz. Because the frequency difference between the channel identification plans is in the range of one to two MHz, the auto fine tuning is inadequate to automatically adjust the tuner to receive various formatted video transmissions
Therefore, a need exists for a method and apparatus for detecting video transmissions that have been formatted based on any one of a plurality of channel identification plans.